In the processing of polymeric resins and other materials, extruders are commonly employed for the plastication, mixing and pumping of such materials. In their simplest form, extruders include a frame designed to be bolted to a concrete floor, a barrel mounted to the frame, and, in the case of a twin screw extruder, two interconnecting bores extending longitudinally from one end of the barrel to the other. A twin screw extruder also includes two intermeshing screws located within the two interconnecting bores and drive means for turning the screws in the same (co-rotating) or opposite (counter-rotating) direction.
Polymeric materials are useful for the fabrication of a variety of films, sheets and molded or shaped articles. As is well-known to those skilled in the art, plastication refers to the softening of a polymeric material to such an extent that it flows freely and will assume any shape. In the case of a polymeric material that is crystalline, plastication is synonymous with melting. In the case of a polymeric material that is amorphous, plastication occurs at or about the glass transition temperature (Tg) thereof.
An extruder screw is shaped generally in the form of an elongated cylinder, and has one or more raised ridges helically disposed thereabout, each of which is a commonly referred to as a flight. A flight may have forward, reverse or neutral pitch, with the degree of pitch varied to accommodate a particular application. The surface of the screw above which the flight is raised is commonly referred to as the root of the screw. When the screw is viewed in cross section, the course of a particular flight, between one point of intersection with a line parallel to the screw axis and the next closest point of intersection of the flight with such line, typically defines a 360° circle. The tip of a flight, which extends toward the perimeter of such circular-shaped cross section, defines a lobe above the root of the screw. The space bounded by the root of the screw and the side walls of any two flights is a channel of the screw. The screw rotates on its longitudinal axis within a barrel or sleeve, which may be generally described as the bore of an annular cylinder.
The screw typically has an initial, feed section which begins the process of conveying solid polymeric material forward within the barrel of the extruder. Polymeric material may be fed into the extruder by means of a hopper which empties into the barrel, or may be metered into the barrel through a feed chute or a side feeder. The direction of travel of the polymeric material in the barrel as it is transported away from the feed port by the screw is known as the downstream direction. In the case of the extrusion of polymer melts, the feed or inlet section of the screw is typically followed, with or without other intervening sections, by a melting section in which partial or complete plastication of the polymeric material occurs.
The melting section of the screw is typically followed, with or without other intervening sections, by a metering section which functions to pump the material, as extrudate, out through the downstream end of the extruder, which typically contains a die or some other form of restricted orifice. The sections of the extruder and screw through which the polymeric material travels before it reaches the die are considered to be upstream from the die.
With respect to a twin screw extruder, two screws are said to be intermeshing if a flight of one screw is disposed within a channel of the other screw. In such a configuration, the distance between the axes of each screw is less than the sum of the respective radii of the two screws, when each radius is measured from the axis to the top of the tallest or highest flight of the screw. When, on a pair of screws, a flight has a shape and size such that its fit into a channel in which it is intermeshed is close enough that essentially no extrudable material passes through the space between the flight and channel, the screws are said to be conjugated. Otherwise, the screws are said to be non-conjugated, and the degree of intermeshing in the case of non-conjugation can be varied to an essentially unlimited extent.
Co-rotating screws, even when conjugated, allow for extensive movement of polymeric material laterally from one screw to the other. Mixing is benefited by this movement and it is further enhanced when the screws are not conjugated. The shape of the flights on non-conjugated screws may be arranged to create the passage of polymeric material from one channel into two channels on another screw. Or, when screws are conjugated, or essentially conjugated, certain flights can be designed in a shape such that they wipe each other in the zone of intermeshing but do not wipe the wall of the barrel.
The production of certain specialty films, such as microporous membranes has presented unique requirements in the design of extruders for their production. This is due in large part by the need to introduce a large amount of a solvent or diluent for the polymeric raw material, e.g., polyolefin resin, so that a polymeric solution (which can also be called a polymeric resin solution) is prepared for subsequent extrusion. Microporous membranes are useful as separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. When the microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, the membrane's performance significantly affects the properties, productivity and safety of the battery. Accordingly, the microporous membrane should have suitably well-balanced permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, etc. The term “well-balanced” means that the optimization of one of these characteristics does not result in a significant degradation in another.
As is known, it is desirable for the batteries to have a relatively low shutdown temperature and a relatively high meltdown temperature for improved battery safety, particularly for batteries exposed to high temperatures under operating conditions. Consistent dimensional properties, such as film thickness, are essential to high performing films. A separator with high mechanical strength is desirable for improved battery assembly and fabrication, and for improved durability. The optimization of material compositions, casting and stretching conditions, heat treatment conditions, etc. have been proposed to improve the properties of microporous membranes.
In general, microporous polyolefin membranes consisting essentially of polyethylene (i.e., they contain polyethylene only with no significant presence of other species) have relatively low meltdown temperatures. Accordingly, proposals have been made to provide microporous polyolefin membranes made from mixed resins of polyethylene and polypropylene, and multi-layer, microporous polyolefin membranes having polyethylene layers and polypropylene layers in order to increase meltdown temperature. The use of these mixed resins and the production of multilayer films having layers of differing polyolefins can make the production of films having consistent dimensional properties, such as film thickness, all the more difficult.
U.S. Pat. No. 5,573,332 proposes a screw element for a screw-type extrusion machine. The screw elements are helical and have varying pitch directions. Lengthwise mixing is obtained by the screwing in opposite directions, whereas crosswise mixing is attained by the elongated wedge of the flank arc. This crosswise flow is a typical continuous shear flow, which is primarily a dispersive mixing operation. Dividing the flow into various partial flows, recirculation and offset combination do not take place. The extruder proposed is for use in preparing a polymer melt and does not relate to the field of polymer solution extrusion.
U.S. Pat. No. 6,062,719 proposes a co-rotating multiple-screw extruder comprising first and second intermeshing screws of more than one flight. The first screw comprises first and second segments paired with first and second segments of the second screw, respectively. On the first segment of the first screw, the height of the first flight is less than the height of the second flight and on the second segment of the second screw, the height of the first flight is less than the height of the second flight and screws for use in such extruder. The extruder proposed is for use in preparing a polymer melt and does not relate to the field of polymer solution extrusion.
U.S. Publication No. 20050013192 proposes a kneading disk having a plurality of disk elements having flight tips arranged at a helix angle E in a direction supporting main streams of a resin. The flight tips of every two adjoining disks have a clearance formed therebetween. The resin is kneaded by undergoing dispersion and distribution without having any excessive temperature elevation in approximately three kinds of streams, i.e. its main streams flowing along the flight tips, its back streams through the clearances and its tip riding streams flowing over the flight tips. The reference discloses a continuous or “rotor”-type screw segment in the “dispersion” region of the extruder for improved melt-shearing in that region. When distribution or “stirring” in needed, a discontinuous or “disk-type” segment having disk elements arranged along a screw axis and flight tips arranged discontinuously and helically in parallel to the screw axis is employed. Polymer flowing counter-currently in the regions between the flight tips (see, e.g., FIG. 7) increases polymer residence time to increase mixing uniformity. With conventional screw segments, the L/D value is small and multiple segments are needed to get good dispersion. This however leads to a problem since, at the interface between two segments in registry, what is effectively produced is a lobe that is twice as long as the interior lobes. This abruptly changes the “pitch” of the flight of lobes. Moreover, the total number of lobes is reduced by the number of segment interfaces. All of these effects serve to reduce the amount of beneficial countercurrent polymer flow.
JP7-216118A discloses a battery separator formed from a porous film comprising polyethylene and polypropylene as indispensable components and having at least two microporous layers each with different polyethylene content. The polyethylene content is 0 to 20% by weight in one microporous layer, 21 to 60% by weight in the other microporous layer, and 2 to 40% by weight in the overall film. The battery separator has relatively high shutdown-starting temperature and mechanical strength. Since this is a “dry” process, the resins are combined as a polymer melt and then extruded.
WO 2004/089627 discloses a microporous polyolefin membrane made of polyethylene and polypropylene comprising two or more layers, the polypropylene content being more than 50% and 95% or less by mass in at least one surface layer, and the polyethylene content being 50 to 95% by mass in the entire membrane. The membrane is made in a wet process, where polymer and a plasticizer are combined by melt blending in a double screw mixer for example. Generally, it is advantageous to combine the polymer first in an inlet stage where the polymer resins can be blended or distributed amongst themselves before adding the plasticizer.
WO 2005/113657 discloses a microporous polyolefin membrane having conventional shutdown properties, meltdown properties, dimensional stability and high-temperature strength. The membrane is made using a polyolefin composition comprising (a) composition comprising lower molecular weight polyethylene and higher molecular weight polyethylene, and (b) polypropylene. This microporous polyolefin membrane is produced by a so-called “wet process”.
As those skilled in the art will plainly recognize, extruder screw design requirements for extruding polymer melts differ greatly from those relating to polymer solutions. While much work has been conducted with respect to polymer melts, this work largely fails to translate to the field of polymer (particularly polyolefin) solution extrusion. Since polyolefin solutions behave differently from polymer melts, those skilled in the art recognize that there is no expectation that a combination of extruder screw segments used for extruding a polymer melt will yield satisfactory performance when extruding a polymer solution. As may be appreciated by those working in the field of polymeric solution extrusion, a counter current flow of the solvent or diluent phase in the extruder can be (and generally is) undesirable. As such, it is desirable to have no significant amount of solvent (preferably none) in the inlet stage of an extruder, since even a small amount of solvent would interfere with polymer blending as a result of the much lower viscosity of the solvent compared to the polymer.
A further problem relating to the extrusion of polymer solutions involves the fact that the knowledge base relating thereto is limited. While it is generally recognized that single and twin-screw extruders can be used, information as to which particular extruder segments or combinations of segments have utility is very limited.
JP 2003-053821 discloses a wet process for manufacturing a microporous film where a polyolefin solution is extruded through a twin-screw extruder and each screw contains at least one of (a) a normal screw-notch screw element, (b) a reverse screw-notch screw element, and (c) a collar. This arrangement is said to benefit the mixing of different kinds and molecular weight polymers. As may be appreciated, the problems identified with respect to U.S. Publication No. 2005/0013192 (too little countercurrent flow of polymer, leading to shorter residence time in the extruder and, consequently, incomplete kneading) are addressed by the introduction of a reverse-pitch segment.
JP Publication Nos. 8-109268, 8-120093, 8-164518, 8-224735, 8-245798 and B-109268 each relate to the field of polymeric solution extrusion. While having an upstream pressure greater than the pressure at the point of solvent injection may be proposed, no teaching as to how this may be achieved is disclosed within any of the aforementioned publications.
Despite these advances in the art, there remains a need for improved extrusion systems capable of producing high quality microporous polyolefin membranes and other films or sheets from polymer solutions.